5

I know you can do: $hash('foo') and $$foo and also $bar[$foo], what are each of these things called?

BoltClock
  • 700,868
  • 160
  • 1,392
  • 1,356
Johnny
  • 1,963
  • 4
  • 21
  • 24

1 Answers1

18

  • $hash('foo') is a variable function.
    $hash may contain a string with the function name, or an anonymous function.

    $hash = 'md5';

// This means echo md5('foo');
// Output: acbd18db4cc2f85cedef654fccc4a4d8
echo $hash('foo');

  • $$foo is a variable variable.
    $foo may contain a string with the variable name.

    $foo = 'bar';
$bar = 'baz';

// This means echo $bar;
// Output: baz
echo $$foo;

  • $bar[$foo] is a variable array key.
    $foo may contain anything that can be used as an array key, like a numeric index or an associative name.

    $bar = array('first' => 'A', 'second' => 'B', 'third' => 'C');
$foo = 'first';

// This tells PHP to look for the value of key 'first'
// Output: A
echo $bar[$foo];

The PHP manual has an article on variable variables, and an article on anonymous functions (but I didn't show an example above for the latter).

BoltClock
  • 700,868
  • 160
  • 1,392
  • 1,356
  • Variable array keys are pretty staple. I've included the PHP manual links for variable variables and variable functions. – BoltClock Sep 05 '10 at 07:57
  • 1
    Things get really funny if you use the extended variable syntax. This allows stuff like `${'prefix_' . $name}` or even funnier things like `${$var = 'var'}` (this is `'var'`). – NikiC Sep 05 '10 at 08:17
  • 3
    +1: "Variable function" calls can also be used on objects that declare an `__invoke` magic method (Since PHP 5.3). So it can handle string function names, the return from `create_function()`, a `Closure` object (which is created when you use anonymous functions since 5.3) or a generic object which declares a `__invoke` method. It's too bad you can't use it on other callback types (`array('classname', 'methodname')`)... – ircmaxell Sep 05 '10 at 08:37